The popularization of a genre...
Doom is the game that put FPS' on the map. Sure they were FPS's before Doom but none had the impact, popularity, political attention and cult following it has had up to now.
Doom puts you in the shoes of a space marine on a lunar base that is being overun by zombies and demons. To survive you will need to shoot your way through the base and even hell itself using guns ranging from shotguns to machineguns to rocket-launchers. There reallys is no story to speak of in this game but this game wasn't intented for storytelling. It's all about killing off hundreds of imps, demons, flying brains and other horrors from hell. The game starts off and it's right to the action and doesn't stop until the end (there will literally only be about 30 secs between each pack of monsters).Doom set the standards for weapons in a FPS. After Doom every FPS had a shotgun, machine-gun, rocket launcher, chaingun and BFG ( or some deviation of these).
At the time of it's release, Doom was at the cutting edge of graphics technology. With 3D environments with multiple floors in the same level (a floor above another which was unheard of at this time.) and detailed sprites it brought a level or realism and gore unparalled at that time.
Sounds were also vey advanced with gun shots and monster groans sounding very realistic. The midi music really helped putting you in that killing mode as well.
Doom also launched the FPS' mutliplay revolution. LAN deathmatches were as frantic as multiplayer action got.
Doom is a timeless classic and FPS' would not be at the forefront of PC gaming as they are today it not for this incredibly fungame.